8 Sivan (Nasso)

Rashi on Bamidbar5,12 says that the reason the halachos of Sotah are written right after the halachos of Kehuna is because Hashem says "If don't bring Matnos Kehuna to the Kohen I swear that you will need to bring your wife to the kohen because she'll be a Sotah. Why,she has Bechirah and she won't sin? How does your doing an Aveirah force your wife to do an Aveirah of adultery (or at least Yichud, which is assur Mideoraysa here since she's married) that Hashem SWEARS that you will have to bring her to the Kohen as a Sotah?